1. Field of the Invention
This invention relates, in general, to numerically controlled digital oscillators and more specifically to frequency multiplier/divider digital oscillators.
2. Description of Background Art
The traditional means of generating an output signal as a function of an input signal may be accomplished by either a frequency counter/divider (FCD) or a numerically controlled oscillator (NCO).
The FCD has a frequency transfer characteristic of EQU f.sub.out =f.sub.ref .div.N (1)
where N is the preset variable interval that the reference frequency (f.sub.ref) is stepped at. This creates a problem at higher frequencies where the steps would by very large (e.g. at f.sub.ref =1 MHz, f.sub.out =500 KHz, 333 KHz etc. for N=2, 3 etc.). The FCD then is incapable of having great resolution for higher output frequencies relative to f.sub.ref.
A second type of circuit that can adjust the frequency in this manner is disclosed in my pending application "Low Spurious Numerically Controlled Oscillator Apparatus and Method," Ser. No. 510,504. The NCO has a frequency transfer characteristic of EQU f.sub.out =f.sub.ref .times.(N/2.sup.n) (2)
where N is again a preset variable interval and n is the fixed number of bits in the word defining N. Taking n as a four-bit word the above equation then becomes, EQU f.sub.out =f.sub.ref .times.(N/16) (3)
which gives more control of the stepping interval than the FCD does, but, which lacks variability in the denominator function as the demoninator can only be powers of two (e.g., 2.sup.n =4, 8, 16 etc. for n=2, 3, 4, etc.).
Neither the NCO nor the FCD have the resolution required at high frequencies because of their large step functions and/or lack of variability.